Bacteria resistant to and capable of biochemically transforming both inorganic and organic mercury compounds are widely found in many facultative, enbacterial genera including the enterobacteriaceae, staphylococci, streptococci, and bacillaceae (Summers, 1986). While industrial contamination can explain the occurrence of such genetic loci in water and soil populations (Barkay and Olson, 1986; Barkay, 1987; Nishimura and Kamahai, 1983; Trevors, 1986; Summers, et al., 1978) their frequent occurrence in the fecal flora of humans (Marshall et al, 1981; Gilbert and Summers, 1988) is harder to understand. It has been known for some time that Hg is released from dental amalgams and that there is considerable variation in the ability of humans to excrete this Hg in urine (Clarkson et al, 1988). Very recently, Hahn, et al. (1989) and Vimy et al (in press) have demonstrated that a substantial burden of Hg in oral and gastrointestinal tissues can result from dental amalgam Thus, the Hg in such restorations may select for resistant bacteria among the normal flora of either the mouth or bowel. Since these bacteria chemically transform Hg compounds, they could have a large influence on the bioavailability of Hg released from dental amalgam. Dr. Fritz Lorscheider and his colleagues at the University of Calgary have assessed the specific organ burden arising from dental amalgam restorations in sheep (Hahn et al, 1989) and plan similar studies in a primate model as well as a survey of selected human subjects. I propose, in collaboration with Dr. Lorscheider, to assess the incidence of Hg resistance in the oral and fecal microbial flora of his animal and human subjects and to correlate this phenomenon with amalgam mass, body burden of Hg, and excretion rate of Hg (as measured by Lorscheiders group). If we find that the presence of a highly resistant microbial flora correlates with reduced body burden or enhanced excretion of Hg, future studies could consider experimental amendment of the flora with naturally occurring Hg resistance plasmids to reduce body accumulation of this highly toxic and mutagenic metal. Alternatively, if increased Hg resistance in the flora enhances body burden, these micro-organisms constitute an additional risk-factor for the use of dental amalgam.